1. Field of the Invention
The present invention relates to a method for inspecting capacitors and more particularly to a method for judging the conformity or non-conformity of capacitors based on the charging characteristic at the time when a direct-current voltage is applied to the capacitors.
2. Description of the Related Art
Generally, in order to judge the conformity or non-conformity of capacitors, a method for measuring the insulation resistance of capacitors in such a way that a leakage current (charging current) of capacitors is measured after a direct-current voltage for measurement is applied to the capacitors and the capacitors have been fully charged is known. Usually, conforming capacitors show little leakage current.
Up to now, as such a method for measuring charging current a measurement system specified in JIS C 5102 is known. In this system, because it is required to measure the current value under the condition that capacitors are fully charged, a measuring time of about 60 seconds was needed. However, the need for cost reduction and reliability improvement of electronic equipment requires an improvement of production capacity and quality of electronic parts such as capacitors, and therefore, the conventional measurement system which needs such a long measuring time for each capacitor cannot satisfy such a requirement in the long run.
Therefore, so far, as a simple inspection method the conformity or non-conformity of capacitors was judged by measuring the current value in a fixed time after a direct-current voltage was applied to the capacitors and comparing the measured current value with a threshold value. However, in such an inspection method, because it is judged whether the measured current value is below the threshold value or not, the voltage is required to be applied till the measured current value becomes greater than the threshold value, and accordingly a long time of a few seconds to tens of seconds was needed.
It is an object of the present invention to provide an inspection method in which the conformity or non-conformity of capacitors can be judged in a short time.
In order to attain the above object, the present invention provides a method for inspecting capacitors in which the conformity or non-conformity of capacitors is judged by the charging current at the time when a direct-current voltage is applied to the capacitors, the method comprising the steps of establishing a threshold current value I0 of the capacitors, formulating an evaluation function using the logarithmic value of the difference between the measured current value m(t) of a dielectric polarization component of the capacitors and the above threshold current value I0 or the difference between their logarithmic values, and judging the conformity or non-conformity of the capacitors based on whether the above evaluation function n(t) results in a line curved upward or downward with respect to a change of logarithmic time.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.